Ziploc has recently come out with large size bags (for blankets, jackets, closet storage, etc). I haven't used anything but the Space Bags and the old "seal up the ziploc baggie most of the way, smash out the air, then close" trick. But once you get it smashed, rolled and sealed, stuffing it under something should keep it from filling. Just make sure it's not hard to pull out if it does re-inflate!
http://ziploc.com/?p=b8


A complete side-note, I love their twistloc containers (although a few of them haven't been air tight, I'm leaning toward the dishwasher as the culprit). <img src="/images/graemlins/smile.gif" alt="" />
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Experience is a hard teacher because she gives the test first, the lesson afterwards.